Tracing the netstack

TODO( Improve ergonomics of enabling tracing on the system netstack.

You can enable tracing for the netstack by setting the enable_netstack2_tracing GN arg to true. Note that this flag is mutually exclusive with use_netstack3; Netstack3 does not currently emit trace events.

Ensure that your fx set includes --with-base //bundles/tools, otherwise Netstack will fail to connect to trace_manager at initialization time.

Make sure to rebuild and reboot your emulator or OTA/flash your device to ensure the system picks up the changes.

How to run a trace

You can use the ffx trace subcommand (see docs) to run a trace on a running Fuchsia system (e.g. an emulator, smart display, or NUC) that is connected to your development host machine. For example:

ffx trace start --categories net,kernel:sched --duration 10

This will record a trace of all categories in the kernel:sched and net categories. (kernel:sched includes detailed scheduler information, and net includes all events emitted by the netstack.) You might want to record this trace while doing something that exercises the netstack on the Fuchsia device; for example, running iperf3.

How to view the trace results

When you run ffx trace, it will produce a trace file that is viewable in Perfetto.